Time to reach a new steady state after changes of positive end expiratory pressure

To assess the time interval required to reach a new steady state of oxygenation-, ventilation-, respiratory mechanics- and hemodynamics-related variables after decreasing/increasing positive end expiratory pressure (PEEP).

In 23 patients (group 1) with acute respiratory distress syndrome (ARDS), PEEP was decreased from 10 to 5 cmH2O and, after 60', it was increased from 5 to 15 cmH2O. In 21 other ARDS patients (group 2), PEEP was increased from 10 to 15 cmH2O and, after 60', decreased from 15 to 5 cmH2O. Oxygenation, ventilation, respiratory mechanics and hemodynamic variables were recorded at time 5', 15', 30' and 60' after each PEEP change.

When PEEP was decreased, PaO2, PaO2/FiO2, venous admixture and arterial oxygen saturation reached their equilibrium after 5'. In contrast, when PEEP was increased, the equilibrium was not reached even after 60'. The ventilation-related variables did not change significantly with PEEP. The respiratory system compliance, when PEEP was decreased, significantly worsened only after 60'. Hemodynamics did not change significantly with PEEP. In the individual patients the change of oxygenation-related variables and of respiratory system compliance observed after 5' could predict the changes recorded after 60'. This was not possible for PaCO2.

We could not find a unique equilibration time for all the considered variables. However, in general, a decremental PEEP test requires far lower equilibrium time than an incremental PEEP test, suggesting a different time course for derecruitment and recruitment patterns.